Cameras and particularly integrated camera optics are nowadays integrated in a large percentage of any electronic devices manufactured, including mobile phones, computers, web cams etc. It is increasingly important that such cameras can be manufactured economically, for example in a parallel process, and that they have as few parts as possible that are mechanically complicated, difficult to manufacture or delicate to handle. Especially for mobile phone applications but also for other applications, there is moreover an increasing demand for cameras that are thin, i.e. the extension in direction of the optical axis is small. Nevertheless, there is also an increasing demand on the resolution that should be achieved by such integrated cameras.
For economical reasons the components of the optical devices, for example lens modules, are often produced on a wafer-scale. A wafer or a wafer stack is thereby produced in an automated process and contains a plurality of congeneric components, as e.g. lens modules, which are individualized in a subsequent process step by separating them from the wafer or wafer stack.
The optical devices, amongst them lens modules, also contain an optoelectronic unit with an image sensor which defines an image sensor plane on which image sensor elements are arranged. Also this optoelectronic units can be manufactured on a wafer-scale as described above. This type of manufacturing processes are well known in the state of the art and e.g. described in the patent publication WO 2009/076 786.
Lens modules comprise an arrangement of one or more lenses or lens parts which serve for the guiding and distribution of the incident light on the image sensor plane of the optoelectronic unit of the camera. Such lens modules have a fix focus which is laid out to match with the distance between the lens module and the image sensor plane of the optoelectronic unit once assembled in the optical device. The fix focus is thereby defined by the flange focal length (FFL), which corresponds to the distance between the last physical plane of the lens (or rather objective) which is the last wafer plane, i.e. that one directed to the sensor in present case, and the focal plane on the side opposite to the object to be imaged, i.e. on the sensor side. Hence the flange focal length refers to a back FFL In order to achieve a high image sharpness, the focal plane and the image sensor plane have to be congruent. I.e., using fix focus lens modules in a large scale production process of optical devices for cameras all lens modules must have a constant FFL.
However, due to fabrication tolerances the lens modules of a wafer or wafer stack assembly and/or the lens modules of different wafers or wafer stack assemblies to some degree have variable FFL-values. I.e., the FFL-value of the lens modules rather follows a normal distribution as e.g. shown in FIG. 3. It is clear that lens modules with an FFL which lies off the center of this normal distribution have to be rejected as the focus plane of the lens module would lie far-off the image sensor plane. However, for an economical production of such optical devices it is essential that as few rejects occur as possible. To reduce the mentioned fabrication tolerances the production process can only be improved to a limited degree. Hence, other ways have to be found to reduce the amount of rejected components.
It is known from the state of the art that lenses are assembled into a barrel and mount and are then focused after assembly on the image sensor. This approach has the drawback of additionally assembly costs, which are caused by the focusing step. A further disadvantage is the large camera footprint since the barrel/mount solution is usually bigger. Further, this method has the risk of so-called “foreign material” on the image sensor. This, because particles of the barrel/mount could fall onto the sensor during the focusing operation.
It is also known from the state of the art to assemble a lens at a fixed focus distance, e.g. by means of a bottom spacer or a mount with a fixed height for all lenses. This method overcomes the disadvantages of the above mentioned approach but has the risk that if the lenses in a production batch have a certain distribution in their FFL's (caused by regular fabrication tolerances), many of the assembled lenses would not be in focus and hence the yield could be quite low.